1. Field of the Invention
The present invention relates to a mobile device provided with a solar battery.
2. Description of the Related Art
With improvements in the light-to-power conversion efficiencies of solar batteries, mobile devices, such as mobile phone terminals, equipped with solar batteries for energy saving are becoming widely used. Although some of such mobile devices use solar batteries as direct sources for supplying power-supply voltages, many typically have rechargeable batteries that are charged with electrical outputs of the solar batteries.
FIGS. 16 and 17 are diagrams showing examples of the configurations of simplified charging circuits. FIG. 16 shows an example of a simplified charging circuit when an output voltage of a solar battery 1 is higher than an output voltage of a rechargeable battery 2 when it is fully charged. In the example of FIG. 16, a voltage regulator 3 is connected between the solar battery 1 and the rechargeable battery 2.
FIG. 17 shows an example of a simplified charging circuit when an output voltage of a solar battery 1 is lower than an output voltage of a rechargeable battery 2 when it is fully charged. In the example of FIG. 17, the output voltage of the solar battery 1 is increased by a step-up circuit 4 to be higher than the output voltage of the rechargeable battery 2 when it is fully charged. The output voltage of the solar battery 1, the output voltage being increased by the step-up circuit 4, is supplied to the rechargeable battery 2 through a charging control circuit 5, so that the rechargeable battery 2 is charged.
When a solar battery is included in, for example, a mobile phone terminal, the maximum size of a solar battery panel that has the light-receiving surface of the solar battery is about 40 mm×70 mm. The light-to-power conversion efficiencies of currently available solar battery panels are about 10 to 15%. In general, the brightness (illuminance) of light that the solar battery panels receive from the Sun is about 100000 lux under the blazing summer sun and is about 30000 lux near the window when it is slightly overcast or it is sunny.
Thus, the illuminance of light (sunlight) received by the solar battery panel varies depending on the season and the weather, and as shown in FIG. 18, output current of the solar battery 1 varies greatly depending on the illuminance. As shown in FIG. 19, charging current from the charging circuit using the solar battery 1 to the rechargeable battery 2 also varies greatly depending on the brightness (illuminance) of light received by the solar battery panel.
When the rechargeable battery is charged using the solar battery under a condition as described above (including the size and the conversion efficiency of the solar battery panel), the charging current is about 30 to 70 mA, as can be seen from FIG. 19.
When the capacity of the rechargeable battery used for the mobile phone terminal is assumed to be, for example, 800 to 1000 mAh, it takes 12 to 33 hours until the rechargeable battery is fully charged from an empty state. Even though solar-battery-based charging is in many cases used as auxiliary charging, it is still preferable that the charging be performed efficiently and effectively.
In view of the foregoing situation, for example, Japanese Unexamined Patent Application Publication No. 10-239464 discloses a technology for controlling a device by using illuminance detecting means for detecting ambient brightness on the basis of an output of a solar battery. Japanese Unexamined Patent Application Publication No. 61-226396 discloses an artificial satellite having a pair of sun detectors provided on a single mounting base on a solar battery panel so as to perform attitude control.